In some situations, it is desirable to monitor emissions such as the exhaust from smoke stacks, engines, industrial processes, etc. to determine the presence and concentration of particular gases and/or aerosols. Sensors employed for such purposes are often subjected to harsh conditions, for example, high temperature and/or corrosive environments. However,.current commercially available gas sensors typically have designs which suffer from temperature limitations and are susceptible to corrosion. Consequently, these commercially available sensors are usually not suitable for use in such harsh environments. In addition, commercially available sensors often have further drawbacks, for example, sluggish response time, large sensor size, inconsistent quality and performance, performance degradation over time, high power requirements and high cost.
The present invention provides a sensor for detecting the presence or absence of selected substances in an atmosphere such as gases, chemical species, aerosols, etc. Some embodiments of the present invention sensor can operate in high temperature and corrosive environments, can be small in size with low power requirements, can have a fast response time, can be low cost, and can be manufactured with consistent quality and provide consistent performance.
One embodiment of the present invention is a gas sensor which includes a p-n junction for subjecting under a reverse electrical bias. A gas sensitive conductive layer is formed across the p-n junction for providing an alternative conductive path across the p-n junction. The conductivity of the conductive layer in the presence of a selected gas is different than in the absence of the selected gas, wherein the conductivity of the conductive layer is indicative of the presence or absence of the selected gas.
In preferred embodiments, the conductive layer has a level of conductivity that vanes with varying concentrations of the selected gas such that the conductivity is indicative of the concentration of the selected gas. The conductive layer can be formed from inorganic, organic, or a composite of organic and inorganic materials. A voltage source is electrically connected to opposite sides of the p-n junction for providing the reverse electrical bias. A measurement device measures electrical properties across the conductive layer. The electrical properties can be any of current, resistance, capacitance and impedance. The p-n junction is formed from n-type and p-type semiconductor regions that are adjacent to each other. In one embodiment, one of the regions is formed within the other region. In another embodiment, one region is over the other region. In such an embodiment, at least one opening can extend through at least a portion of the n-type and p-type regions to expose the p-n junction within the opening. At least a portion of the conductive layer is disposed within the opening across the p-n junction. The conductive layer further extends over at least a portion of the n-type and p-type regions surrounding the at least one opening. In yet another embodiment, a cavity is formed below the n-type and p-type regions to thermally isolate the sensor from the substrate. A heating arrangement can be provided for heating the gas sensor.
The present invention also provides a sensor including a p-n junction for subjecting under a reverse electrical bias. A conductive layer is formed across the p-n junction for providing an alternative path across the p-n junction. The conductivity of the conductive layer in the presence of a selected substance in an atmosphere is different than in the absence of the selected substance, wherein the conductivity of the conductive layer is indicative of the presence or absence of the selected substance.
In preferred embodiments, the conductive layer has a level of conductivity that varies with varying concentrations of the selected substance such that the conductivity is indicative of the concentration of the selected substance. A voltage source is electrically connected to opposite sides of the p-n junction for providing the reverse electrical bias. A measurement device measures electrical properties across the conductive layer. The electrical properties are any of current, resistance, capacitance and impedance.
The present invention also provides a method of sensing a selected gas with a sensor including subjecting a p-n junction of the sensor under a reverse electrical bias. A gas sensitive conductive layer extends across the p-n junction for providing an alternative conductive path across the p-n junction. The conductivity of the conductive layer in the presence of the selected gas is different than in the absence of the selected gas. Electrical properties are measured across the conductive layer to determine the presence or absence of the selected gas.
In preferred embodiments, the conductive layer has a level of conductivity that varies with varying concentrations of the selected gas. The concentration of the selected gas is determined based on the level of conductivity of the conductive layer. Any of current, resistance, capacitance and impedance is measured across the conductive layer. In some embodiments, the sensor is heated with a heating arrangement to a desired operating temperature.
The present invention also provides a method of sensing a selected substance in an atmosphere with a sensor including subjecting a p-n junction of the sensor under a reverse electrical bias. A conductive layer extends across the p-n junction for providing an alternative conductive path across the p-n junction. The conductivity of the conductive layer in the presence of the selected substance in the atmosphere is different than in the absence of the selected substance. Electrical properties are measured across the conductive layer to determine the presence or absence of the selected substance.
In preferred embodiments, the conductive layer has a level of conductivity that varies with varying concentrations of the selected substance. The concentration of the selected substance is determined based on the level of conductivity of the conductive layer. Any one of current, resistance, capacitance and impedance is measured across the conductive layer. In some embodiments, the sensor is heated with a heating arrangement to a desired operating temperature.